Vodacom mystery charges - poor accounting or ****?

Vodacom either has a poor billing system or is running a bit of a ****. I have a contract with them and I am VERY particular in paying my invoice within a day or two of receiving it. Despite this, they send me a notice for an outstanding amount. Also, a large amount appears on the statement, usually around three times my monthly account. This has happened three times in the past and has just happened again, which leads me to think that is not just a random occurrence. This time, as in the past, I have had a heated discussion over the phone where I ask to be shown where I haven't paid. I was promised a statement showing the problem, which I never received. Now, I didn't even get a monthly invoice, just another demand email. Has anyone else experienced this? If so, it seems to be a ***** business practice.